drm: apple: Set swap timestamps to sane values for Adaptive Sync

Setting these timestamps to a dummy value worked fine for enabling
a fixed 120 Hz mode on the MacBook Pros, however doing so causes
Adaptive Sync displays to simply switch between full and minimum
refresh rates. Setting these timestamps based on the swap pacing
seems to fix this, and makes the display's refresh rate match
the incoming swap rate.

Note that the names and values are best-guess only. These seem
to work fine for driving VRR displays, but may still be incorrect.

Signed-off-by: James Calligeros <[email protected]>

Author: chadmed

drivers/gpu/drm/apple/iomfb_template.c

@@ -1299,6 +1299,15 @@ int DCP_FW_NAME(iomfb_modeset)(struct apple_dcp *dcp,
 	return 0;
 }
 
+/*
+ * DCP timestamps are expressed in system timer ticks. Approximate
+ * this by converting from ktime nanoseconds to 24 MHz ticks.
+ */
+static u64 ns_to_mach(u64 ns)
+{
+	return ns * 3 / 125;
+}
+
 void DCP_FW_NAME(iomfb_flush)(struct apple_dcp *dcp, struct drm_crtc *crtc, struct drm_atomic_state *state)
 {
 	struct drm_plane *plane;
@@ -1415,12 +1424,14 @@ void DCP_FW_NAME(iomfb_flush)(struct apple_dcp *dcp, struct drm_crtc *crtc, stru
 
 	if (has_surface && dcp->use_timestamps) {
 		/*
-		 * Fake timstamps to get 120hz refresh rate. It looks
-		 * like the actual value does not matter, as long  as it is non zero.
+		 * TODO: ascertain with certainty what these timestamps
+		 * are. They are something to do with presentation timing,
+		 * but that is all we know for sure. These values seem to
+		 * work well with VRR.
 		 */
-		req->swap.ts1 = 120;
-		req->swap.ts2 = 120;
-		req->swap.ts3 = 120;
+		req->swap.unk_pres_ts1 = ns_to_mach(ktime_get_ns());
+		req->swap.unk_pres_ts2 = ns_to_mach(ktime_to_ns(dcp->swap_start));
+		req->swap.unk_pres_ts3 = req->swap.unk_pres_ts1;
 	}
 
 	/* These fields should be set together */

drivers/gpu/drm/apple/iomfb_template.h

@@ -18,14 +18,14 @@
 #include "version_utils.h"
 
 struct DCP_FW_NAME(dcp_swap) {
-	u64 ts1;
-	u64 ts2;
+	u64 unk_pres_ts1;
+	u64 unk_pres_ts2;
 
 	u64 unk_10;
 	u64 unk_18;
 	u64 ts64_unk;
 	u64 unk_28;
-	u64 ts3;
+	u64 unk_pres_ts3;
 	u64 unk_38;
 
 	u64 flags1;